On the average, two tsunamis occur per year throughout the world which inflict damage near the source. Approximately every 15 years a destructive, ocean-wide tsunami occurs.

Do all large undersea earthquakes generate tsunamis?

No. The earthquake must cause significant vertical deformation of the seafloor in order for a tsunami to occur. Tsunamis are more likely to happen from shallower and larger earthquakes than deeper or smaller ones.

How big is a tsunami?

Tsunamis range in size from inches to over a hundred feet. In deep water (greater than 600 feet/180 meters), tsunamis are rarely over 3 feet (1 meter) and will not be noticed by ships due to their long period (time between crests). As tsunamis propagate into shallow water, the wave height can increase by over 10 times.

Tsunami heights vary greatly along a coast. The waves can be amplified by shoreline and bathymetric (sea floor) features. A large tsunami can flood low-lying coastal land over a mile from the coast.

How long do tsunamis last when they happen?

Since a tsunami consists of a series of waves, the danger can last for many hours. Large tsunamis are generally recorded for a few days following the event.

How fast do tsunamis travel?

Tsunami Wave Depth vs. Speed of Travel

Depth

Velocity

feet

meters

mph

km/h

23,000

7,000

586

943

13,000

4,000

413

713

6,500

2,000

194

313

650

200

99

159

165

50

49

79

33

10

22

36

Tsunami velocity depends on the depth of water through which it travels (velocity equals the square root of the product of the water depth times the acceleration of gravity). Tsunamis travel approximately 475 mph (765 km/h) in 15,000 feet (4,500 meters) of water. In 100 feet (30 meters) of water the velocity drops to about 40 mph (64 km/h).

Do all large earthquakes, greater than magnitude 7, generate tsunamis?

No, only those which induce large vertical sea floor displacements, or those that trigger landslides which displace significant amounts of sea water, trigger tsunamis.

Seismic waves travel about 100 times faster than tsunami waves. This means, we can detect and study the earthquake much more quickly than the tsunami itself.

Can strike-slip (horizontal motion) earthquakes trigger tsunamis?

Yes, approximately 15% of all damaging tsunamis were triggered by strike-slip earthquakes. However, this type of earthquake is less likely to trigger a tsunami than one with vertical motion. Tsunamis are generated by associated landslides or motion of a sloping bathymetric (sea floor) feature and normally affect regions near the source only.

A tsunami travels from the central Aleutian Islands to Hawaii in about five hours and to California in about six hours, or from the Portugal coast to North Carolina in about 8½ hours.

Where should I go in the event of a tsunami warning or large, local earthquake?

Know evacuation routes and potential hazard zones for your area. Most coastal communities have an evacuation plan and designated safe areas. This information can be obtained from your local emergency official. If no tsunami hazard zone has been established or you don't know what it is, as a rule of thumb move to 100 feet (30 meters) above sea level or 1 mile (1.6 km) inland. If the tsunami hazard zone has been delineated for your area, stay out of that area.

What do I do if I'm in a boat at sea or in a harbor during a tsunami event?

Mariners in deep water (600 feet or greater/180 meters) should stay at sea. Those in shallow water or harbors should move to deep water if there is enough time and weather conditions are suitable.

What are the natural warning signs for a tsunami?

Severe ground shaking from local earthquakes may cause tsunamis. As a tsunami approaches shorelines, water may recede from the coast, exposing the ocean floor, reefs and fish. Abnormal ocean activity, a wall of water, and an approaching tsunami create a loud "roaring" sound similar to that of a train or jet aircraft.

If you experience any of these phenomena, don't wait for official evacuation orders. Immediately leave low-lying coastal areas and move to higher ground.

What should I do in the event of a tsunami warning?

If you are in school and you hear there is a tsunami warning, follow the advice of teachers and other school personnel.

If you are at home and hear there is a tsunami warning, you should make sure your entire family is aware of the warning. Your family should evacuate your house if you live in a tsunami evacuation zone. Move in an orderly, calm and safe manner to the evacuation site or to any safe place outside your evacuation zone. Follow the advice of local emergency and law enforcement authorities.

If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground. DO NOT wait for a tsunami warning to be announced. Stay away from rivers and streams that lead to the ocean as you would stay away from the beach and ocean if there is a tsunami. A tsunami from a local earthquake could strike some areas before a tsunami warning could be announced.

Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally-generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground.

High, multi-story, reinforced concrete hotels are located in many low-lying coastal areas. The upper floors of these hotels can provide a safe place to find refuge should there be a tsunami warning and you cannot move quickly inland to higher ground. Local Civil Defense procedures may, however, not allow this type of evacuation in your area. Homes and small buildings located in low lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning.

Offshore reefs and shallow areas may help break the force of tsunami waves, but large and dangerous waves can still be threat to coastal residents in these areas. Staying away fro all low-lying coastal areas is the safest advice when there is a tsunami warning.